The invention relates to a substrate for soil improvement that has a water-storing property, a method for producing the substrate, and use of the substrate.
As soil improvers on the basis of natural products, for example, graft copolymers of starch are used that, however, are decomposed over the course of time of a few weeks. For the use as a soil water storage, it is important that the latter is not too quickly decomposed. Lignin is a biopolymer which, in comparison to polysaccharides such as starch or cellulose, is more stable with respect to microbial decomposition.
DE 198 28 483 A1 discloses a product as a soil improver or for improvement of plant growth medium that contains a lignin sulfonate gel as an important component. This product is said to bind water and ionic nutrient materials. For producing the lignin sulfonate gel of lignin sulfonates, reference is being had, inter alia, to the generally known crosslinking with formaldehyde (e.g., U.S. Pat. No. 3,864,276 and U.S. Pat. No. 4,332,589). The generally known crosslinking reaction is performed in the cited publications for crosslinking of the inherently water-soluble lignin sulfonates.
The product that is disclosed in DE 198 28 483 A1 is said to have a capacity of binding water; however, this type of crosslinking of lignin sulfonates, as is well-known, leads to sluggishly reacting gels that are swellable only modestly. The water absorption capacity of the lignin sulfonate gels is not specified in more detail.
DE 27 03 812 A1 discloses a crosslinked lignin gel and a method for producing crosslinked lignin gels that are used as controlled-release carriers for different pesticides. For producing the gel, preferably alkaline lignin is crosslinked with formaldehyde, glutar aldehyde or epichlorohydrin. When doing so, gels are formed that absorb water in an amount of 3 to 11 times their own weight.
The publication DE 27 03 812 A1 discloses that the aldehyde-crosslinked gel has a small dry surface area and an almost non-existing water-free pore structure. Accordingly, the gel is suitable for controlled release of pesticides but is entirely unsuitable as a water storage in the soil.
DE 26 23 663 A1 discloses a pesticide mass with gel carrier and a method for its preparation. This invention concerns an improved carrier on the basis of lignin for controlled release of organic water-insoluble pesticides. Crosslinking of lignin is realized by means of epichlorohydrin. As in DE 27 03 812 A1, the product, because of its minimal porosity of only 1.8 m2/g, is unsuitable as a water storage. Crosslinking of alkaline lignin with epichlorohydrin is disclosed also in U.S. Pat. No. 4,131,573.
Crosslinking of lignin with diepoxides, however for producing flocculant agents, is disclosed in U.S. Pat. No. 3,857,830. Here, crosslinking of lignin is carried out with diepoxides in aqueous alkaline medium and organic solvents wherein water-insoluble epoxides are reacted in water-insoluble organic solvents. As crosslinking agents, C-linked diepoxides, diglycidyl ethers, and diglycidyl amines are used.
The use of bifunctional epoxides, especially of poly (ethylene glycol) diglycidyl ether, for producing hydrogels based on lignin is disclosed in M. Nishida, Y. Uraki, Y. Sano, Proc. ISWPC Vol II, 43-46, 2001, and M. Nishida, Y. Uraki, Y. Sano, Bioresource Technol. 2003, 88, 81-83. Here, lignins obtained by acid hydrolysis were used and their swelling behavior examined in aqueous organic solvents. The swelling capacity of the gels was comparatively minimal. H. Yamamoto, M. Amaike, H. Saitoh, Y. Sano, Mat. Sci. Eng. C7 2000, 143-147, disclose the preparation of hydrogels from a mixture of Kraft lignin, phenol and formaldehyde. The water absorption capacity of these gels was minimal and was not the focus of consideration. A further possibility for synthesis of lignin hydrogels resides in copolymerization between acrylamide and polyvinyl alcohol with lignin (W. K. El-Zawawy, Polym. Adv. Technol. 2005, 16, 48-54). The water absorption capacity of the described gels is less than 10 g water per g of dried gel substance.
The object of the invention resides in that a substrate on the basis of lignin for soil improvement having a water-storing property, a method for producing, and its use are to be provided, which substrate binds the stored water over a longer period of time and releases it, as needed, into the soil wherein the lignin basis should not react sluggishly and be swellable.